Environmental Engineering Reference
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success. Of 12 pairs of birds exposed to an overspray, eight nests failed completely. In contrast,
only two of 14 burrows sprayed with carbaryl exhibited complete nest failure. During a telephone
interview conducted after the study, two landowners reported complete disappearance of owls after
a carbofuran overspray (three and eight burrows affected, respectively) at colonies that were outside
the study area. Burrow reoccupancy in 1987 was less frequent after the 1986 use of carbofuran than
after the use of other insecticides.
The signifi cance of these fi ndings is increased when one considers that the researchers did not
direct or control any of the spraying, in other words they collected the data from an
a posteriori
study design. In addition, the statistical analysis was very conservative, in that it allowed for possible
biases among owl colonies, such as a farm effect, even though no such biases were documented.
The impact on nest success was related to distance of the spraying from the burrow. There was
some indication that spray events occurring even at more than 50 metres from a burrow were also
detrimental to reproductive success (0.05 < P < 0.1). These data further suggest that the effect of
carbofuran was due to toxicity rather than food removal. Another insecticide, notably carbaryl, did
not appear to affect owl reproduction despite its removal of insect food. There were too few data to
be certain about the safety of a third insecticide in use, chlorpyrifos.
In response to these study results, the Canadian Regulatory Authorities initially tried to limit the
risk to burrowing owls through labelling restrictions and farmer education although there is evi-
dence that these attempts were largely ineffective (Trowsdale Mutafov 1992; Mineau 1993). They
cancelled several uses of the liquid formulation in 1995 in order to completely eliminate exposure
to burrowing owls.
8.4.2.4 Brusnyk and Westworth 1987
Birds are the focus of most directed fi eld studies because of the taxa's extreme sensitivity to carbo-
furan. Consequently, the fi nding of other vertebrate wildlife is often considered incidental. This
study is an exception in that the researchers studied only small mammals.
Brusnyk and Westworth chose two study sites, a dry pasture and another site with high inter-
spersion of wetlands. They established one control and one treated plot on each site, and used a
ground sprayer to apply carbofuran at 132 g ai/ha to the treated plot. The investigators did not pro-
vide the size of the sprayed plots or any measure of pesticide deposit.
The researchers live-trapped small mammals for six consecutive days pre-spray and three consecutive
days post-spray. They also conducted late-season trapping about fi ve weeks post-spray. The researchers
collected some individuals between 16 and 20 hours post-spray for brain cholinesterase assays.
Meadow vole (
Microtus pennsylvanicus
) numbers decreased in both habitats after the spray,
although the authors considered the decline signifi cant only in the pasture habitat, where numbers
fell by about 85%. It appears that the investigators did not analyse these data statistically, and the
basis for rejecting the vole decline in the wetland habitat is therefore unclear. In contrast, total
numbers of deer mice remained constant post-spray.
Turnover rates of marked individuals on both treated and control plots were high, and survival
rates were based on small numbers of recaptured individuals. Survival, defi ned as the proportion
of marked animals recaptured, was much lower for young deer mice and for all voles in treated
plots. Survival of adult deer mice did not appear to be affected. However, in post-spray plots the
researchers found a signifi cantly higher proportion of male deer mice in non-reproductive status
relative to expected numbers from the control plots.
Mean brain cholinesterase levels in collected deer mice did not differ signifi cantly between
control and treated plots, but there was evidence that some (males especially) were exposed to car-
bofuran based on the variances in the mean brain cholinesterase levels reported (F
18.65, df
15,
P < 0.05). The authors did not provide individual data to substantiate this.
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